A passive optical network (PON) comprises an optical line terminal (OLT) connected to multiple optical network units (ONUs) in a point-to-multi-point network. New standards have been developed to define different types of PONs, each of which serves a different purpose. For example, the various PON types known in the related art include a Broadband PON (BPON), an Ethernet PON (EPON), ten Gigabit-Ethernet PON (10G-EPON) a Gigabit PON (GPON), ten-Gigabit PON (XG-PON), and others.
An exemplary diagram of a typical PON 100 is schematically shown in FIG. 1. The PON 100 includes N ONUs 120-1 through 120-N (collectively referred to as ONUs 120) connected to an OLT 130 via a passive optical splitter 140 and the optical fiber. In a GPON, for example, traffic data transmission is achieved using a GPON encapsulation method (GEM) over two optical wavelengths, one for the downstream direction and another for the upstream direction. Thus, downstream transmission from the OLT 130 is broadcast to all the ONUs 120. Each ONU 120 filters its respective data according to pre-assigned labels (e.g., GEM port-IDs in a GPON). The splitter 140 is 1 to N splitter, i.e., capable of distributing traffic between a single OLT 130 and N ONUs 120.
In most PON architectures, the upstream transmission is shared between the ONUs 120 in a TDMA based access, controlled by the OLT 130. TDMA requires that the OLT 130 first discovers the ONUs and measures their round-trip-time (RTT), before enabling coordinated access to the upstream link. With this aim, the OLT 130, during a ranging state, tries to determine the range between the terminal units (i.e., ONUs 120) to find out at least the RTT between OLT 130 and each of the ONUs 120. The RTT of each ONU 120 is necessary in order to coordinate a TDMA based access of all ONUs 120 to the shared upstream link. During a normal operation mode, the range between the OLT 130 to the ONUs 120 may change over time due to temperature changes on the fiber links (which results with varying signal propagation time on the fiber). Thus, the OLT 130 continuously measures the RTT and adjusts the TDMA scheme for each ONU accordingly.
As schematically shown in FIG. 2, an OLT 200 operable, for example, in a GPON or XG-PON includes an electrical module 210 and an optical module 220. The electrical module 210 is responsible for the processing of received upstream burst signals and generating downstream signals. The electrical module 210 typically includes a network processor and a media access control (MAC) adapter designed to process and handle upstream and downstream signals according to a respective PON standard.
The optical module 220 in most cases is implemented as a small form-factor pluggable (SFP) transceiver that receives optical burst signals sent from ONUs and transmits continuous optical signals to ONUs. The reception and transmission of signals is over two different wave lengths. For example, in a GPON, in the downstream direction, the optical module 220 generates an optical signal of 1480 nm to 1500 nm (as referred to 15XY) and in the upstream direction receives optical signals between 1260 nm and 1360 nm (also referred to as 13XY in GPON).
The optical module 220 includes a laser driver diode 221 coupled to a transmit laser diode that produces optical signals based on the electrical signals provided by the laser diode driver 221. The module 220 also includes a limiter amplifier 222 coupled to a receive photodiode that produces current in proportion to the amount of light of the optical input burst signal. The limiter amplifier 222 generates two current levels indicating if a received burst signal is ‘1’ or ‘0’ logic value.
The receiver/transmitter optical elements (i.e., a photodiode and laser diode) are realized as a bidirectional optical sub-assembly (BoSa) module 223 that can transmit and receive high rate optical signals. The optical module 220 also includes a controller 224 that communicates with the electrical module 210 through the I2C interface and performs tasks related to calibration and monitoring of the transceiver.
Vendors of the OLTs typically develop and fabricate the electrical module 210 of the OLT, where the optical module 220 is often an off-the-shelve transceiver, such as SFP, XFP and the like. Thus, the interface between the electrical module 210 and the optical module 220 is a standard interface being compatible with any type of SFP transceiver. As illustrated in FIG. 2, the interface includes wires for receive (RX) data, transmit (TX) data, TX-enabled signal, RX-Reset signal, and I2C for interfacing between the electrical module 210 and the controller 224. The I2C interface is a relatively slow serial interface with a data rate of up to 4 Mb/sec. In contrast, the RX data and TX data interfaces are high speed interfaces where the data rate of signals over these interfaces is as the data rate of the PON (e.g., 1 Gb/sec in a GPON).
In certain PON configurations, a dedicated ONU is connected to the PON to perform maintenance and service availability applications. For example, a dedicated ONU can be utilized as part of a protection mechanism. Other examples include, a dedicated ONU can be utilized to perform optical time-domain reflectometer (OTDR) analysis in the PON, measure RTT values, detect optical faults, and so on. Examples for utilization of dedicated ONUs in the PON can be found in co-pending U.S. patent application Ser. Nos. 12/648,885 and 13/189,935 assigned to common assignee and are hereby incorporated by reference.
Optical faults and their locations in the PON can be detected using optical time-domain reflectometers (OTDRs). The principle of an OTDR includes injecting, at one end of the optical fiber, a series of optical pulses into the optical fiber under test and also extracting from the same end of the fiber, light that is scattered (Rayleigh backscatter) or reflected back from points along the fiber. The strength of the return signals is measured and integrated as a function of time and may be plotted as a function of fiber length. The results may be analyzed to determine the fiber's length, overall attenuation, optical faults, such as breaks, and to measure optical return loss.
The OTDR measurements can be performed in the PON using an out-of-band, an in-band, or a dedicated wavelength technique. Out-of-band testing requires stopping the normal operation of the network and verifying the fiber using external OTDR tools. This can be performed using, for example, wavelengths and test pulses that are separate and independent from and different from other wavelengths used to carry customer service traffic.
The in-band OTDR testing is performed when the network is operational. However, such a testing requires dedicated OTDR testing signals. The OTDR testing signals utilized in conventional in-band OTDR solutions are either AM modulated or FM modulated. However, such signals can be transmitted only during a test period of the PON, during which data signals are not transmitted to the ONUs. Other OTDR solutions utilize a dedicated upstream wavelength for measuring reflection from the fiber.
These OTDR techniques are performed using an external testing device that could be either an OTDR tool or a dedicated optical unit connected in the PON and adapted to perform OTDR measurements.
In conventional solutions, a dedicated ONU is connected through an optical fiber (which may be a dedicated fiber, the PON's fiber, or combination thereof) to the OLT. As a result, the OTDR measurements performed using the dedicated ONU should take into account the delay induced by the optical fiber connecting the OLT to the ONU. The induced delay is typically determined through a ranging process.
In addition, the dedicated ONU should be within a small optical distance from the OLT. However, this is not always the case within systems utilized to detect optical failures. By way of example, the system discussed in the Ser. No. 13/189,935 application, where an OLT is connected to a dedicated ONU (i.e., collocated ONU) through a splitter and a dedicated optical fiber to form an optical link utilized for transmitting signals for the purpose of failure detections.